The phenomenon known as "ablative photodecomposition" was developed in 1982 by Srinivasan et al. using a 193 nm ArF laser and is described in U.S. Pat. No. 4,784,135. At that time, the phenomenon was explained in terms of the excitation of bound molecular states to repulsive and ionic states by single photon absorption of 6.5 ev laser energy. The ablated fragments carry most of the energy away from the substrate surface thus minimizing the conversion of radiant energy into heat. The surface of the ablated substrate, be it a polymer or biological tissue, does not experience charring, carbonization or burning. Both the scientific literature and patents based on this discovery explicitly taught that using laser wavelengths higher than 193 nm to accomplish ablation would obviate the beneficial results of ablation with 193 nm laser radiation and result in burning of the underlying substrate material. Srinivasan explicitly prescribed using an unfocussed 193 nm laser beam.
In 1983, Gruen et al. discovered that a focused 308 nm laser beam produced ablative photodecomposition without charring of tissue and explained this new finding on the basis of multiphoton processes. Although each 4 ev photon of 308 nm radiation does not contain sufficient energy to reach repulsive and ionic molecular states, two-photon absorption can result in ejection of molecular products. However, the simultaneous absorption of two photons can occur only at intensities of 0.1 to 1 GW/cm.sup.2, achievable only by focussing the 308 nm beam of an excimer laser. U.S. Pat. No. 4,686,979 based on this discovery points out that tissue can be removed at a location far away from the laser beam source by transmitting the laser light through a fiber optic cable and that the effects on tissue of unfocussed 193 nm laser radiation and focussed 308 nm laser radiation are the same. Thus, it had heretofore been known that an unfocussed 193 nm laser beam could be used in ablative photodecomposition without charring of tissue and it was believed that using a laser beam for this purpose above 193 nm in wavelength required focussing of the beam. It was thought that the 40% decrease in photon energy going from 193 nm to 308 nm laser radiation was compensated for by focussing the 308 nm radiation on the substrate.
The 193 nm excimer laser commonly used in ablative photodecomposition employs fluorine and argon gas to create an ArF molecule in an excited electronic state by virtue of an electrical discharge in the gas. The output of such a laser typically is 100 mJ with a pulse length of 10 nsec and repetition rate of 100 Hz. The beam diameter typically is 1 cm.sup.2. The intensity of the laser beam is thus: ##EQU1##
The fluorine used in the excimer laser is a highly toxic gas. Although there have been advances in the materials used in manufacturing these types of lasers, the gases employed in the laser must be changed periodically requiring pump-down of the laser chamber and occasional major reconditioning of the chamber because of corrosion. In addition to the high toxicity and highly corrosive nature of the gases employed in excimer lasers, this approach also has the undesirable characteristic of producing ozone requiring the enclosing of the laser beam and frequent flushing of the enclosure.
The use of a solid state laser system would avoid the aforementioned problems provided sufficient power was available. Commercially available Nd:YAG lasers present an alternative to the use of the excimer laser in that they afford 50 Hz repetition rates, 1 cm.sup.2 beam cross sections, and 1200 mJ output power at the 1064 nm fundamental frequency. Doubling the fundamental frequency and using the sum frequency generated with the 532 nm doubled frequency and 1064 mm fundamental frequency affords 350 mJ of tripled 355 nm radiation. Nonlinear optics crystals known as a beta barium borate (BBO) crystals are frequently used in this type of laser system because of their high UV transmission characteristics. These nonlinear optics crystals decrease the number of photons in the laser beam, and increase the energy per photon in the beam. Using a BBO crystal, one would expect .about.50% efficiency from the sum frequency generation using doubled 532 nm and tripled 355 nm radiation to generate on the order 175 mJ of 213 nm radiation. To generate 175 mJ at 213 nm using a 1 cm.sup.2 BBO crystal, the largest defect-free size currently available, exceeds the optical damage limit of the crystal material, which is on the order of 35 mJ/cm.sup.2 for a 10 nsec laser pulse at 213 nm. The power handling capability of the BBO crystal thus limits the power of the output laser beam.
The present invention addresses the aforementioned limitations of the prior art by providing a BBO crystal matrix array capable of handling high laser beam power without damaging the individual crystals. This invention further contemplates generating an unfocussed 213 nm laser beam using a commercially available, solid state Nd:YAG laser, which beam is particularly adapted for use in single photon ablative photodecomposition such as of polymers and biological tissue. This invention further contemplates generating an unfocussed 266 nm laser beam using a commercially available, solid state Nd:YAG laser, which beam is particularly adapted for transmission through a fiber optic cable and use in single photon ablative photodecomposition. Additionally, this invention contemplates using a crystal matrix array in conjunction with a solid state Nd:YAG laser to generate laser radiation at a selectable wavelength in the range of 210 nm to 400 nm, which is the optimal wavelength to achieve single or multi photon ablation of a given polymer or biological tissue with specific absorptive properties.